As is well known, normal speech contains silent intervals which form part of the intelligence that is conveyed. Periods of silence occur between sentences, phrases, words and within words. Ordinarily, silent intervals are accepted by the listener as natural attributes of speech. The silence, however, accounts for a significant portion of the speech pattern. When a speech signal is coded for transmission over a communication channel or for storage in memory, the code sequences corresponding to silence occupy segments of the coded signal which can be eliminated or utilized for other purposes. It is, of course, necessary to reproduce the silence intervals in their proper locations in the speech pattern to understand the message. But the coding for silence can be simplified so that the stored or transmitted digital signal is made more compact. In this way, the efficiency of the communication system is increased.
Many schemes are available for digitally coding speech signals. Direct conversion of sampled speech to binary form may be accomplished by pulse code modulation. The conversion process includes quantizing each speech sample to one of a set of discrete levels and coding the selected quantized level. Adaptive forms of pulse modulation in which the quantization of the speech signal samples is adapted to the level of the input signal provides improved performance with fewer bits per digital sample. In such adaptive arrangements, the step-size in the quantizer is permitted to vary so as to match the statistical characteristics of the input signal. Differential pulse modulation systems encode the difference between input signal samples rather than the samples themselves to improve coding efficiency. These coding arrangements, however, do not accurately distinguish between active and inactive portions of the input signal. Consequently, the silence periods in the signal adversely affect the efficiency of intelligence transmission.
U.S. Pat. No. 4,280,192 issued to E. W. Moll on July 21, 1981, discloses an arrangement to minimize the space for digital storage of analog information in which an input analog signal such as speech is converted into a stream of continuously variable slope delta modulation (ADM) codes. A voice operated switch detects the beginning of a pause interval when the analog signal falls below a predetermined level. The pause is timed by a counter which is stopped when the speech signal rises above a predetermined level. A special pause code including timing information is then inserted into the digital code stream. The voice operated switch and the timing arrangements remove repetitive codes of the pause intervals of the analog signal.
Voice operated switches, as is well known in the art, are designed to operate at a low rate to prevent syllable clipping and include apparatus that inhibits the detection of short silent intervals. Consequently, the Moll system utilizes a compensating delay to facilitate the insertion of pause codes in their proper locations in the data stream. But the delay inherent in the voice operated switch makes it difficult to detect short periods of silence that occur at higher than word rate.
U.S. Pat. No. 4,053,712 issued to A. Reindl on Oct. 11, 1977, discloses an adaptive digital coder decoder in which special coded bit patterns are substituted for idle patterns in a CVSD output bit stream to reduce the overall bit rate. Idle pattern detection is accomplished by converting the CVSD output to an analog signal and comparing the analog signal to fixed amplitude thresholds. The regeneration of the analog speech signal to detect silence requires an additional decoder arrangement that adds to the cost of the coding. While direct amplitude thresholds make the arrangement relatively free from rate restriction in silence detection, there is increased sensitivity to noise. As a result, it is difficult to distinguish fricatives from silence and the arrangement is more susceptible to speech clipping. It is an object of the invention to provide improved digital coding adapted to economically eliminate silence intervals without rate restriction.